The present invention relates to a structure of a wrist having three degrees of freedom for use with an industrial robot.
A wrist coupled to the end of the arm of an industrial robot is required to have suitable motions for carrying out a desired job in three dimensional space. Thus, as shown in FIG. 1, an entire wrist B coupled to an arm A must be rotated in a direction shown by an arrow a, the angle of a tool C mounted on the wrist must be adjusted or tilted as shown by a reference character b, and the tool itself must be rotated as shown by a reference character c. In order to meet the above requirements, three drive shafts must be inserted into the interior of the arm supporting the wrist, so that the outer dimension and weight of the arm disadvantageously increase. Therefore, the applicants of this application together with other coinventors have proposed a new wrist mechanism disclosed in Japanese Utility Model Application No. 58-44671 filed Mar. 26, 1983. The wrist mechanism is provided with two sets of differential speed reduction devices within a wrist. The devices are arranged symmetrically of and perpendicularly to the axis of an arm. The adjustment of the angle of the wrist and the rotation of a tool are carried out through the differential speed reduction devices, so that input axes for the devices can be made slender and hence the arm can be smaller while improving the accuracy of a mechanism.
Such a prior art wrist mechanism will be described with reference to FIG. 2.
Reference number 1 designates an arm, and reference number 2 designates a first input shaft for rotating the whole of a wrist, the first input shaft 2 being coupled to a wrist seat 4 supported by a bearing 3 mounted on the end of the arm 1. Reference number 5 designates a frame integral with the wrist seat 4. Reference number 6 designates a wrist cover having a spherical shape to be divided into two semi-spherical covers 6a and 6b along a line oblique to the axis of the arm 1. The divided covers 6a and 6b may be rotated along a division plane through a bearing 8. Reference number 9 designates a second input shaft passing through the interior of the first input shaft 2 and extending to the interior of the wrist cover 6. Reference number 10 designates a third input shaft passing through the interior of the second input shaft, and reference numbers 11 and 12 designate respective bevel gears coupled to the second and third input shafts. Reference number 13 designates a speed reduction shaft supported by brackets 14, 14 mounted on the frame 5, and arranged within the wrist cover perpendicularly to the shaft axis of the arm. Reference number 15, 16 designate differential speed reduction devices each mounted opposite ends of the speed reduction shaft 13.
The differential speed reduction device 15 comprises an input shaft 15a, a drive wheel 15b, a ball bearing 15c, a flexible ring 15d, a fixed wheel 15e, and an output shaft 15f. The input shaft 15a is rotatably mounted on the speed reduction shaft 13 and has a bevel gear 17 meshing with the bevel gear 11. The drive wheel 15b of an elliptical shape is fixedly connected to the input shaft 15a. The flexible ring 15d has a plurality of spline teeth on the outer periphery thereof, and is slidably mounted on the outer periphery of the drive wheel 15b through the ball bearing 15c. The fixed wheel 15e fixed to the bracket 14 and the output shaft 15f rotatably supported by the bracket 14, are each provided with spline teeth meshing with the spline teeth of the flexible ring 15a and having a number of teeth slightly different from that of the flexible ring 15d. In operation, when the input shaft 15a and hence the drive wheel 15b integral therewith is rotated, the longer diameter side portions of the drive wheel 15b push the flexible ring 15d against the fixed wheel 15e and also against the output shaft 15f to thereby mesh both spline teeth with each other. The meshing portions are moved sequentially along the spline teeth while the flexible ring 15d is driven to rotate. Therefore, the flexible ring 15d is rotated at a lower speed reduced in proportion to the difference of the number of spline teeth from that of the fixed wheel 15e, and the output shaft 15f meshing with the flexible ring 15d is rotated at a lower speed reduced in proportion to the difference of the number of spline teeth therebetween.
The differential speed reduction device 16 is constructed in the same way as that of the above mentioned differential speed reduction device 15, in which an input shaft 16a is provided with a bevel gear 18 which meshes with the third input shaft 10.
Reference number 19 designates a tilting shaft mounted on the wrist cover 6b through a bearing 20. The tilting shaft 19 is aligned to as for the axis thereof to pass the intersection point O between the axes of the arm 1 and the speed reduction shaft 13, and is coupled to the output shaft 15f of the differential speed reduction device 15. Reference number 21 designates a tool rotation shaft disposed concentrically and interiorly of the tilting shaft 19 and coupled to the output shaft 16f of the differential speed reduction device 16 through bevel gears 22 and 23. Reference number 24 designates a flange mounted on the tool rotation shaft 21 for mounting a tool.
In effect, the first input shaft 2 driven at a reduced speed by a drive section and passing through the interior of the arm 1, rotates the wrist seat 4, while the second and third input shafts 9 and 10 are disposed within the arm 1 operatively at a high speed. The second input shaft 9 causes the differential speed reduction device 15 to operate via the bevel gears 11 and 17, and in turn causes the tilting shaft 19 to move about the speed reduction shaft 13. At this time, since the wrist cover 6 is divided obliquely relative to the axis of the arm, and the tilting shaft 19 is arranged at one side of the wrist cover 6b, so the tilting shaft 19 causes the cover 6b to turn around through the bearing 7 in the same direction as that of the tilting shaft 19, and also causes the cover 6a to turn around through the bearing 8 by a component force along the direction of the speed reduction shaft. Thus, the tilting shaft 19 tilts as a low speed while retaining a perpendicular relation to the speed reduction shaft.
Further, with the third input shaft 10 driven at a high speed, the differential speed reduction device 16 causes the bevel gear 22 to rotate at a low speed through the bevel gears 12 and 18. The bevel gear 23 meshing with the bevel gear 22 causes, the tool rotation shaft 21 and hence the flange 24 for mounting a tool, to be driven into rotation at a low speed.
The wrist mechanism described above, however, can not be always in good condition because the two divided wrist covers 6a and 6b may cause malfunction. More in detail, among various relative positions of the shafts for driving a wrist, there is a relation when the axis of the first input shaft for effecting rotation shown by an arrow a in FIG. 1 and the axis of the tool rotation shaft for effecting rotation shown by an arrow c in FIG. 2 coincide with each other. At this time, since the bearing 20 and the bearing 8 have the same rotation axis, the wrist cover 6 shown in FIG. 2 comes to a state free to rotate.
The divided wrist covers 6a and 6b at the state shown in FIG. 2 can be rotated as one integral body by hand along the rotary direction with respect to the bearings 20 and 8, and can also be stopped by hand at any position as desired.
In this free state, it is assumed here that the tilting or bending plane defined by a tilting or bending direction b of the second input shaft and a division or separated plane 6c of the cover 6 are caused to intersect at a right angle, by giving a suitable rotation amount to the covers 6a and 6b. When the second input shaft is actuated under the above condition, then the cover 6 is brought into a dead point so that the function thereof can not be obtained and the breakage may happen.
The critical condition explained above have been introduced by using hand. in practice, however, there is an ample possibility that such a dead point condition may occur to damage the cover 6. For example, upon application of any external force through cables of a tool or the like attached to the end of the arm, the cover 6 under the above mentioned free state may be led to the dead point condition.